Rapid shutdown of photovoltaic systems

ABSTRACT

A photovoltaic system includes groups of solar cells that can be switched in and out of the photovoltaic system. In response to detecting initiation of rapid shutdown, a control circuit controls a switch device to switch out a group of solar cells to lower the output voltage of the photovoltaic system below a safety level. In response to detecting a release trigger that indicates resumption of normal operation, the control circuit controls the switch device to switch back the group of solar cells to restore the output voltage of the photovoltaic system to a normal operating level. Solar cells may be switched out by disconnecting them from the photovoltaic system and switched back by reconnecting them into the photovoltaic system. Solar cells may also be switched out by shorting them out of the photovoltaic system and switched back in by removing the short.

TECHNICAL FIELD

Embodiments of the subject matter described herein relate generally tosolar cells. More particularly, embodiments of the subject matter relateto rapid shutdown of photovoltaic systems.

BACKGROUND

Solar cells are well known devices for converting solar radiation toelectrical energy. A solar cell has a front side that faces the sunduring normal operation to collect solar radiation and a backsideopposite the front side. Solar radiation impinging on the solar cellcreates electrical charges that may be harnessed to power an externalelectrical circuit, such as a load.

A photovoltaic panel, which is also referred to as a photovoltaicmodule, comprises a string of solar cells that are packaged together ona common support structure, such as a frame. A photovoltaic system maycomprise one or more photovoltaic panels that form an array of solarcells. Photovoltaic systems may be installed in a residential housing,commercial building, or power plant as a green energy source. Because aphotovoltaic panel generates power as long as its solar cells receivesolar radiation, the photovoltaic panel's output voltage may pose ahazard to firefighters or other personnel who may have to be near thephotovoltaic panel in the event of an emergency. The 2017 NationalElectric Code (NEC) Section 690.12 introduces a requirement for rooftopphotovoltaic systems to limit controlled conductors to 80V or lesswithin the array boundary and to 30V or less outside of the arrayboundary within 30 seconds of initiation of a rapid shutdown of thephotovoltaic system.

BRIEF SUMMARY

In one embodiment, a photovoltaic system includes groups of solar cellsthat can be switched out of the photovoltaic system and switched backinto the photovoltaic system. In response to detecting initiation ofrapid shutdown of the photovoltaic system, a control circuit controls aswitch device to switch out a group of solar cells to lower the outputvoltage of the photovoltaic system below a safety level. In response todetecting a release trigger that indicates resumption of normaloperation, the control circuit controls the switch device to switch backthe group of solar cells to restore the output voltage of thephotovoltaic system to a normal operating level. Solar cells may beswitched out by disconnecting them from the photovoltaic system andswitched back in by reconnecting them back into the photovoltaic system.Solar cells may also be switched out by shorting them out of thephotovoltaic system and switched back in by removing the short.

These and other features of the present invention will be readilyapparent to persons of ordinary skill in the art upon reading theentirety of this disclosure, which includes the accompanying drawingsand claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a photovoltaic system in accordancewith an embodiment of the present invention.

FIG. 2 shows a schematic diagram of a control circuit in accordance withan embodiment of the present invention.

FIG. 3 shows a state diagram of operating a photovoltaic system withsafety shutdown in accordance with an embodiment of the presentinvention.

FIG. 4 shows a flow diagram of a method of operating a photovoltaicsystem with safety shutdown in accordance with an embodiment of thepresent invention.

FIG. 5 shows a photovoltaic panel in accordance with an embodiment ofthe present invention.

FIG. 6 shows a photovoltaic panel in accordance with another embodimentof the present invention.

FIG. 7 shows the photovoltaic panel of FIG. 5 with an additional switch,in accordance with an embodiment of the present invention.

FIG. 8 shows the photovoltaic panel of FIG. 5 with two switches and twodiodes in accordance with an embodiment of the present invention.

FIG. 9 shows a photovoltaic panel in accordance with another embodimentof the present invention.

FIG. 10 shows a photovoltaic panel in accordance with another embodimentof the present invention.

FIG. 11 shows a photovoltaic panel in accordance with another embodimentof the present invention.

FIG. 12 shows the photovoltaic panel of FIG. 5 with solar celldisconnection and shorting in accordance with an embodiment of thepresent invention.

FIG. 13 shows a photovoltaic panel in accordance with another embodimentof the present invention.

FIG. 14 shows the photovoltaic panel of FIG. 6 with a rapid shutdowncontrol circuit for switching out solar cells in accordance with anembodiment of the present invention.

FIG. 15 shows a photovoltaic panel in accordance with another embodimentof the present invention.

FIG. 16 shows the photovoltaic panel of FIG. 9 with a rapid shutdowncontrol circuit in accordance with an embodiment of the presentinvention.

FIG. 17 shows a flow diagram that schematically illustrates operation ofa photovoltaic system in accordance with an embodiment of the presentinvention.

FIG. 18 shows a schematic diagram of a photovoltaic system in accordancewith an embodiment of the present invention.

FIG. 19 shows a schematic diagram of a photovoltaic system in accordancewith an embodiment of the present invention.

FIG. 20 shows a schematic diagram of a photovoltaic system in accordancewith an embodiment of the present invention.

FIG. 21 shows a schematic diagram of a photovoltaic system in accordancewith an embodiment of the present invention.

The use of the same reference label in different drawings indicates thesame or like components.

DETAILED DESCRIPTION

FIG. 1 shows a schematic diagram of a photovoltaic system 180 inaccordance with an embodiment of the present invention. In the exampleof FIG. 1, the photovoltaic system 180 comprises a photovoltaic panel100, a rapid shutdown control circuit (“control circuit”) 150, and aphotovoltaic inverter 160. The photovoltaic system 180 may have aplurality of photovoltaic panels and inverters, but only one of each isshown in FIG. 1 for clarity of illustration. The control circuit 150 maybe integrated in the photovoltaic panel 100 and receive power from oneor more solar cells of the photovoltaic panel 100.

In the example of FIG. 1, the photovoltaic panel 100 comprises aplurality of solar cells 120 (i.e., 120-1, 120-2, 120-3, . . . , 120-n)and 130 (i.e., 130-1, 130-2, 130-3, . . . , 130-n). In general, thenumber of solar cells that a photovoltaic panel may have will depend onthe particulars of the application. In one embodiment, a solar cell 120or 130 is a hyper cell, such as those commercially-available from theSunPower™ Corporation. Generally speaking, a hyper cell comprises aplurality of serially-connected solar cells. Hyper cells may beincorporated in P-series photovoltaic panels, which arecommercially-available from the SunPower™ Corporation. Hyper cells andP-series photovoltaic panels are disclosed in commonly-assigned PCTApplication No. PCT/US2015/032472, which is published as PCT publicationNo. WO2015183827A2, incorporated herein by reference in its entirety. Itis to be noted that embodiments of the present invention are not limitedto hyper cells and P-series photovoltaic panels. Embodiments of thepresent invention are applicable to other photovoltaic panel and solarcell designs.

In the example of FIG. 1, the solar cells 120 form a first group (orsub-circuit) of parallel-connected solar cells across the nodes 122 and121. The positive end of the first group of solar cells is connected tothe node 121 and the negative end of the first group of solar cells isconnected to the node 122. The solar cells 130 form a second group ofparallel-connected solar cells across the nodes 123 and 125. Thepositive end of the second group of solar cells is connected to the node123 and the negative end of the second group of solar cells is connectedto the node 125. In one embodiment, the number of solar cells 120 is thesame as the number of solar cells 130. However, the number ofseries-connected solar cells in a solar cell 120 is larger than thenumber of series-connected solar cells in a solar cell 130. The numberof solar cells in a solar cell 130 is such that the contribution of thesolar cells 130 to the panel voltage during safety shutdown mode isbelow a safety level that complies with the 2017 NEC Section 690.12 orother safety requirement.

In the example of FIG. 1, the first group of solar cells is seriallyconnected to the second group of solar cells of solar cells byconnecting the node 122 to the node 123. The positive end of thephotovoltaic panel 100 at the node 121 is connected to the positive endof the photovoltaic inverter 160, and the negative end of thephotovoltaic panel 100 at the node 125 is connected to the negative endof the photovoltaic inverter 160.

Generally speaking, the output voltage of a photovoltaic panel isreferred to as the panel voltage and the output current of aphotovoltaic panel is referred to as the panel current. The outputvoltage of a photovoltaic system is referred to as the system voltage,which is the sum of the voltage contributions of the photovoltaicpanels. Similarly, the output current of the photovoltaic system isreferred to as the system current. In the example of FIG. 1, with onephotovoltaic panel 100, the panel voltage and panel current are theoutput voltage and output current, respectively, of the photovoltaicsystem 180.

In one embodiment, an initiation event that indicates rapid shutdown ofthe photovoltaic system 180 is triggered when the output current of thephotovoltaic system 180 drops down to zero. In one embodiment, shuttingdown the photovoltaic inverter 160 is an initiation event that initiatesrapid shutdown of the photovoltaic system 180.

The photovoltaic inverter 160 is configured to convert the directcurrent (DC) output of solar cells to alternating current (AC) output(FIG. 1, 161). In the example of FIG. 1, the photovoltaic panel 100 isconnected to the photovoltaic inverter 160 through the control circuit150. More particularly, the node 121 is connected to a node 126, whichis connected to the positive end of the photovoltaic inverter 160.Similarly, the node 125 is connected to a node 127, which is connectedto the negative end of the photovoltaic inverter 160. When thephotovoltaic inverter 160 is shutdown (e.g., powered off or disconnecteddue to an emergency or maintenance), the connection to the photovoltaicinverter 160 causes the panel current to drop to zero.

In the example of FIG. 1, the control circuit 150 comprises a switch 151and a bypass diode 152. The bypass diode 152 has an anode that isconnected to the node 125 and a cathode that is connected to the node124. One end of the switch 151 is connected to the node 124, and theopposing end of the switch 151 is connected to the node 121. Duringnormal operation, the switch 151 is open and the bypass diode 152 isreverse biased. Accordingly, the panel current flows from the node 127,to the node 125, to the node 123, to the node 122, to the node 121, andto the node 126. Closing the switch 151 creates a low resistance paththrough the node 124, the switch 151, and the node 126, therebyswitching out the solar cells 120 across the nodes 121 and 122. That is,in the example of FIG. 1, the solar cells 120 are switched out bycreating an electrical short (“short”) across the nodes 121 and 122. Thesolar cells 120, having been switched out, no longer contribute to thepanel voltage, thereby lowering the panel voltage and thus the systemvoltage. Opening the switch 151 removes the short to switch the solarcells 120 back into the photovoltaic system 180.

In one embodiment, the control circuit 150 is configured to detect aninitiation event that is indicative of rapid shutdown, to place thephotovoltaic panel 100 in safety shutdown mode by lowering the panelvoltage of the photovoltaic panel 100 below a safety level, to remainoperational while the photovoltaic panel 100 is in the safety shutdownmode, to wait for a release trigger from the photovoltaic inverter 160,and, in response to receiving the release trigger, to place thephotovoltaic panel 100 back in normal operation mode by restoring thepanel voltage of the photovoltaic panel 100 to its normal operatinglevel.

In one embodiment, the control circuit 150 is configured to lower thepanel voltage of the photovoltaic panel 100 below the safety level byswitching out the solar cells 120, and to restore the panel voltage ofthe photovoltaic panel 100 back to the normal operating level byswitching back the solar cells 120. In one embodiment, the controlcircuit 150 is configured to detect an initiation of rapid shutdown bydetecting for zero panel current for a predetermined amount of time(e.g., longer than a threshold time). As noted, the switch 151 is openduring normal operation mode. In response to detecting the initiationevent, the control circuit 150 closes the switch 151 to switch out thesolar cells 120 and remove their voltage contribution to the panelvoltage, thereby lowering the panel voltage below the safety level.While the solar cells 120 are switched out, the solar cells 130 continueto generate output power for the control circuit 150 to allow thecontrol circuit 150 to remain operational and wait for the releasetrigger. Powering the control circuit 150 using the solar cells 130during safety shutdown mode advantageously simplifies resumption of thephotovoltaic panel 100 to normal operation. The solar cells 130 andother solar cells that are configured to remain operational duringsafety shutdown mode, e.g., to provide power to a rapid shutdown controlcircuit or other circuits, are also referred to as supply cells.

The photovoltaic inverter 160 may send the release trigger to thecontrol circuit 150 by sending a trigger signal (e.g., ripple voltage,reverse current) on the line connecting the panel voltage to thephotovoltaic inverter 160 or by sending a control signal on a dedicatedconnection 162 (e.g., wired or wireless connection) to the controlcircuit 150. In response to detecting the release trigger signal, thecontrol circuit 150 opens the switch 151 to switch back the solar cells120 and raise the panel voltage back to its normal operating level.

As a particular example, the photovoltaic panel 100 may generate a panelvoltage of 42.8 V when the solar cells 120 and 130 are both connected.By switching out the solar cells 120 during safety shutdown mode, thesolar cells 130 may continue to generate a panel voltage of 1.25V orother suitable panel voltage. In that case, the panel voltage duringsafety shutdown mode is relatively low but is sufficient to keep thecontrol circuit 150 operational. The number of solar cells to switch outand to keep in the string during safety shutdown mode depends on thevoltage outputs of the solar cells and the target safety level.

FIG. 2 shows a schematic diagram of the control circuit 150 inaccordance with an embodiment of the present invention. In the exampleof FIG. 2, the nodes 121 and 124-127 are connected as shown in FIG. 1.More particularly, the control circuit 150 is connected to the paneloutput power (i.e., panel voltage and panel current) by connecting tothe nodes 121 and 125. The cathode of the bypass diode 152 is connectedto the node 124 and the anode of the bypass diode 152 is connected tothe node 125. In the example of FIG. 2, the switch 151 is a field effecttransistor (FET). The switch 151 may also be another type of transistor(e.g., bipolar junction transistor, MOSFET) or another switch device. Inthe example of FIG. 2, one terminal of the switch 151 (e.g., drainterminal) is connected to the node 121 and the other terminal of theswitch 151 (e.g., source terminal) is connected to the node 124. Thecontrol terminal of the switch 151 (e.g., gate terminal) is connected toa controller 201.

In one embodiment, the controller 201 comprises a microcontroller withintegrated data acquisition (e.g., analog to digital converter) andcontrol (e.g., analog and digital input/output) circuits. The controller201 may include firmware for implementing a state machine and otherprogrammed logic. The controller 201 may also comprise discretecircuits.

A power supply 202 provides the supply voltage Vcc to the controller201. The power supply 202 may comprise a DC-DC converter that receivesinput power from the solar cells 130 across the nodes 124 and 125. Usingthe solar cells 130 to provide input power to the power supply 202advantageously allows the control circuit 150 to be operated by solarenergy and simplifies maintenance of the photovoltaic panel 100. In theexample of FIG. 2, the power supply 202 receives the input power fromthe solar cells 130, which remain (i.e., not switched out) in the stringwhen the photovoltaic panel 100 is in safety shutdown mode. Moreparticularly, the power supply 202 continues to receive input power fromthe solar cells 130, thereby providing supply voltage to the controller201 even during the safety shutdown mode. This advantageously allows thecontrol circuit 150 to remain operational during the safety shutdownmode.

In one embodiment, the control circuit 150 monitors the panel voltage byway of a voltage sense circuit comprising the resistors R1 and R2, andmonitors the panel current by way of a current sense circuit comprisingthe resistor R3. More particularly, in the example of FIG. 2, theresistors R1 and R2 form a voltage divider that scales the panel voltageacross the nodes 126 and 127. The controller 201 senses the voltageacross the resistor R2 to monitor the panel voltage. In one embodiment,the controller 201 detects a release trigger signal by monitoring thepanel voltage for a superimposed or added predetermined release triggersignal. For example, the release trigger signal may be a controlledripple voltage or reverse current that is generated by the photovoltaicinverter 160 to indicate resumption of normal operation. Thephotovoltaic inverter 160 may also transmit the release trigger signalto the controller 201 over a dedicated connection 162.

In the example of FIG. 2, the panel current develops a current sensevoltage across the resistor R3. The controller 201 senses the currentsense voltage across the resistor R3 to monitor the panel current. Inone embodiment, the control circuit 201 detects initiation of rapidshutdown by monitoring for zero panel current for a predetermined periodof time. For example, the control circuit 150 may enter safety shutdownmode when the panel current is zero for longer than five seconds, whichindicates that the photovoltaic inverter 160 has been shut down.

In the example of FIG. 2, the controller 201 drives the control terminalof the switch 151. In one embodiment, the controller 201 closes theswitch 151 when the controller 201 detects the initiation eventindicative of rapid shutdown, and opens the switch 151 when thecontroller 201 detects the release trigger signal.

FIG. 3 shows a state diagram 300 of operating a photovoltaic system inaccordance with an embodiment of the present invention. In oneembodiment, the state diagram 300 is implemented by the control circuit150.

In the example of FIG. 3, upon start up, the photovoltaic system 180 isin normal operation mode (state 310). During normal operation mode, thecontrol circuit 150 opens the switch 151 to allow voltage contributionsfrom the solar cells 120 and 130 of the photovoltaic panel 100 (seeFIG. 1) to be available as the panel voltage that is input to thephotovoltaic inverter 160. In one embodiment, rapid shutdown of thephotovoltaic system 180 is initiated when the photovoltaic inverter 160is shutdown, thereby dropping the panel current to zero. The controlcircuit 150 detects the initiation of rapid shutdown when the panelcurrent is zero for longer than a threshold time (e.g., five seconds).The control circuit 150 transitions 301 from normal operation mode(state 310) to safety shutdown mode (state 320) in response to detectingthe initiation event by closing the switch 151 to lower the panelvoltage below a safety level (e.g., below 80V for within the arrayboundary). The photovoltaic inverter 160 may be configured to generatethe release trigger signal when the photovoltaic inverter 160 is poweredback up to resume normal operation. The control circuit 150 transitions302 from safety shutdown mode to normal operation mode in response todetecting the release trigger signal.

FIG. 4 shows a flow diagram of a method 400 of operating a photovoltaicsystem with safety shutdown in accordance with an embodiment of thepresent invention. The method 400 may be performed by the controlcircuit 150. As can be appreciated, the method 400 may also beimplemented using other rapid shutdown control circuits or suitablecomponents without detracting from the merits of the present invention.

In the example of FIG. 4, the control circuit 150 monitors the panelcurrent and panel voltage of the photovoltaic panel 100 (step 401). Whenthe control circuit 150 detects that the panel current dropped to zeroand stays at zero for at least a threshold length of time, indicatinginitiation of rapid shutdown, the control circuit 150 switches out thesolar cells 120 to lower the panel voltage (step 402 to step 403).Otherwise, when the panel current does not drop to zero for at least thethreshold period of time, the control circuit 150 continues monitoringthe panel current and panel voltage to detect initiation of rapidshutdown (step 402 to step 401).

Switching out the solar cells 120 lowers the panel voltage below atarget safety level (step 403). The target safety level depends on thesafety requirement, and may be 80V for within the array boundary or 30Vfor outside of the array boundary in the case of the NEC Section 690.12.In one embodiment, the solar cells 130 serve as supply cells and are notswitched out to continue to provide power to the control circuit 150(step 404).

After switching out the solar cells 120 in response to detectinginitiation of rapid shutdown, the control circuit 150 waits for arelease trigger signal from the photovoltaic converter 160 (step 405).In response to detecting the release trigger signal, indicatingresumption of normal operation, the control circuit 150 restores thepanel voltage to its normal operating level by switching back the solarcells 120 (step 406 to step 407). Otherwise, the control circuit 150continues to wait for the release trigger signal (step 406 to step 405).

Embodiments of the present invention may be implemented by switching outsome but not all solar cells of a photovoltaic panel or by switching outan entire photovoltaic panel, i.e., all of the solar cells of thephotovoltaic panel. Switching out some but not all photovoltaic panelsof a photovoltaic system will lower the output voltage of thephotovoltaic system. Solar cells may be switched out by shorting thesolar cells out of the photovoltaic system, and may be switched back byremoving the short as in the photovoltaic system 180 of FIG. 1. Solarcells may also be switched out by disconnecting the solar cells from thecircuit of the photovoltaic system.

FIG. 5 shows a photovoltaic panel 500 in accordance with an embodimentof the present invention. The photovoltaic panel 500 may be incorporatedin a photovoltaic system to implement rapid shutdown. In the example ofFIG. 5, the photovoltaic panel 500 may be switched out during safetyshutdown mode by disconnecting the photovoltaic panel 500 from thephotovoltaic system.

In the example of FIG. 5, the photovoltaic panel 500 comprises solarcells 520 (i.e., 520-1, 520-2, 520-3, . . . , 520-m). Only some of thesolar cells 520 are labeled for clarity of illustration. In oneembodiment, a solar cell 520 is a hyper cell, and the photovoltaic panel500 is a P-series panel. Hyper cells and P series panels arecommercially available from the SunPower™ Corporation. It is to be notedthat embodiments of the present invention are not limited to hyper cellsand P series panels.

In the example of FIG. 5, the photovoltaic panel 500 includes bus bars510-514. The solar cells between the bus bars 510 and 511 form a firstgroup of solar cells, the solar cells between the bus bars 511 and 512form a second group of solar cells, etc. The diodes D1-D4 are bypassdiodes for the groups of solar cells. More particularly, the first groupof solar cells has the bypass diode D1, the second group of solar cellshas the bypass diode D2, etc. The panel voltage of the photovoltaicpanel 500 is available across a positive node 501 and a negative node502 during normal operation.

In the example of FIG. 5, a switch 505 has a first end that is connectedto the bus bar 514 and a second end that is connected to an anode of abypass diode D5. A wire 503 connects a cathode of the diode D5 to thepositive node 501. During normal operation, the switch 505 is closed,thereby providing the panel voltage of the photovoltaic panel 500 acrossthe nodes 501 and 502. In response to detecting an initiation event thatindicates rapid shutdown of the photovoltaic system, safety shutdownmode is entered by opening the switch 505 to disconnect the photovoltaicpanel 500 from the photovoltaic system and thereby reduce the systemvoltage below the safety level. When the switch 505 is open, any reversecurrent will flow through the diode D5 so that the switch 505 does nothave to block more than the panel voltage of the photovoltaic panel 500.In response to receiving a release trigger signal that indicates end ofthe safety shutdown mode, the switch 505 is closed to thereby reconnectthe photovoltaic panel 500 back into the photovoltaic system.

FIG. 6 shows a photovoltaic panel 500A in accordance with an embodimentof the present invention. The photovoltaic panel 500A is a particularimplementation of the photovoltaic panel 500 of FIG. 5. The photovoltaicpanel 500A is the same as the photovoltaic panel 500 except that thewire 503 is integrated within the photovoltaic panel 500A for safety.

In the photovoltaic panels 500 and 500A, the number and placement of thediode D5 and the switch 505 may be varied depending on theimplementation.

FIG. 7 shows the photovoltaic panel 500 of FIG. 5 with an additionalswitch 506 in accordance with an embodiment of the present invention. Inthe example of FIG. 7, the switches 506 and 505 are closed during normaloperation to provide the panel voltage across the nodes 501 and 502. Inresponse to detecting an initiation event, safety shutdown mode isentered by opening the switches 506 and 505 to disconnect thephotovoltaic panel 500 from the photovoltaic system, and thereby removethe panel voltage contribution of the photovoltaic panel 500. Inresponse to detecting a trigger release signal, the switches 506 and 505are closed to reconnect the photovoltaic panel 500 back to thephotovoltaic system to resume normal operation.

FIG. 8 shows the photovoltaic panel 500 of FIG. 5 with two switches andtwo diodes in accordance with an embodiment of the present invention. Inthe example of FIG. 8, the panel voltage of the photovoltaic panel 500is across a positive node 561 and a negative node 562. In the example ofFIG. 8, the switches 560 and 563 are closed during normal operation toprovide the panel voltage across the nodes 561 and 562. In response todetecting an initiation event, safety shutdown mode is entered byopening the switches 561 and 562 to disconnect the photovoltaic panel500 from the photovoltaic system, and thereby remove the panel voltagecontribution of the photovoltaic panel 500. In response to detecting atrigger release signal, the switches 560 and 563 are closed to reconnectthe photovoltaic panel 500 to the photovoltaic system to resume normaloperation. Because the diodes D60 and D61 are in series and between thestring, the switches 560 and 563 only need to block half the panelvoltage.

FIG. 9 shows a photovoltaic panel 500B in accordance with an embodimentof the present invention. The photovoltaic panel 500B is a particularimplementation of the photovoltaic panel 500 of FIG. 5. The photovoltaicpanel 500B is the same as the photovoltaic panel 500 except for theaddition of another group of solar cells that serve as supply cellsbetween the bus bars 514 and 515. A diode D7 is across the supply cells.The bus bars 510-514 and diodes D1-D4 are otherwise the same as in thephotovoltaic panel 500.

In the example of FIG. 9, the bus bar 510 is connected to the positivenode 531 and the bus bar 515 is connected to the negative node 532. Theswitch 530 is open during normal operation to provide the panel voltageacross the nodes 531 and 532. In response to detecting an initiationevent, safety shutdown mode is entered by closing the switch 530 toshort out the solar cells between the bus bars 510 and 514, therebyremoving their contribution to the panel voltage. In the example of FIG.9, the switch 530 shorts out the bulk of the solar cells of thephotovoltaic panel 500B to limit the panel voltage so that systemvoltage is limited to below a safety level, such as below 80V or 30V. Insome implementations, the panel voltage may be limited to approximately5V to limit the system voltage to 80V, or to 2V to limit the systemvoltage to 30V. The solar cells between the bus bars 514 and 515, whichserve as supply cells, are not shorted out to provide power to acorresponding rapid shutdown control circuit during safety shutdownmode. The diode D7 serves as a protective bypass diode in the event thesolar cells between the bus bars 514 and 515 become shaded during normaloperation.

FIG. 10 shows a photovoltaic panel 500C in accordance with an embodimentof the present invention. The photovoltaic panel 500C is a particularimplementation of the photovoltaic panel 500 of FIG. 5. The photovoltaicpanel 500C is the same as the photovoltaic panel 500B of FIG. 9 exceptthat the supply cells are on the high side.

In the example of FIG. 10, the solar cells between the bus bars 516 and510 serve as supply cells for providing supply voltage to a controlcircuit even when in safety shutdown mode. A diode D8 is across thesolar cells between the bus bars 516 and 510. The bus bars 510-514 anddiodes D1-D4 are otherwise the same as in the photovoltaic panel 500.

In the example of FIG. 10, the bus bar 516 is connected to the positivenode 537 and the bus bar 514 is connected to the negative node 538. Theswitch 536 is open during normal operation to provide the panel voltageacross the nodes 537 and 538. In response to detecting an initiationevent, safety shutdown mode is entered by closing the switch 536 toshort out the solar cells between the bus bars 510 and 514, therebyremoving their contribution to the panel voltage. The solar cellsbetween the bus bars 516 and 510 are not shorted in safety shutdown modeto provide power to a corresponding rapid shutdown control circuit. Thediode D8 serves as a protective bypass diode in the event the solarcells between the bus bars 516 and 510 become shaded during normaloperation.

FIG. 11 shows a photovoltaic panel 500D in accordance with an embodimentof the present invention. The photovoltaic panel 500D is a particularimplementation of the photovoltaic panel 500 of FIG. 5. The photovoltaicpanel 500D demonstrates that the supply cells may be placed in variouslocations within the photovoltaic panel and can be used with additionalswitches. In general, multiple sets of supply cells and differentnumbers of bypass diodes and switches may be employed depending on theapplication.

In the example of FIG. 11, a bus bar 517 is added between the bus bars511 and 512 to accommodate supply cells between the bus bars 517 and512. A diode D9 is connected across the supply cells between the busbars 517 and 512. A cathode of the diode D9 is connected to an anode ofthe diode D2, and an anode of the diode D9 is connected to a cathode ofthe diode D3. The bus bars 510-514 and diodes D1-D4 are otherwise thesame as in the photovoltaic panel 500 of FIG. 5.

In the example of FIG. 11, the bus bar 510 is connected to the positivenode 548 and the bus bar 514 is connected to the negative node 549. Theswitches 546 and 547 are open during normal operation to provide thepanel voltage across the nodes 548 and 549. In response to detecting aninitiation event, safety shutdown mode is entered by closing theswitches 546 and 547 to short out the solar cells between the bus bars510 and 517 and between the bus bars 512 and 514, thereby removing theircontribution to the panel voltage. The solar cells between the bus bars517 and 512, which serve as supply cells, are not shorted in safetyshutdown mode to provide power to a corresponding rapid shutdown controlcircuit. The diode D9 serves as a protective bypass diode in the eventthe solar cells between the bus bars 517 and 512 become shaded duringnormal operation.

FIG. 12 shows the photovoltaic panel 500 with solar cell disconnectionand shorting in accordance with an embodiment of the present invention.In the example of FIG. 12, the bus bar 510 is connected to the positivenode 551. The connection of the bus bar 514 to the negative node 553 iscontrolled by a switch 554. During normal operation, the switch 554 isclosed and the switch 552 is open, thereby providing the panel voltageacross the nodes 551 and 553. In response to detecting an initiationevent, safety shutdown mode is entered by opening the switch 554 andclosing the switch 552. Opening the switch 554 disconnects the solarcells between the bus bars 512 and 514, thereby removing theircontribution to the panel voltage. Closing the switch 552 shorts thesolar cells between the bus bars 510 and 512, also removing theircontribution to the panel voltage. Accordingly, in safety shutdown mode,all of the solar cells of the photovoltaic panel 500 are switched out ofthe photovoltaic system. When the switch 554 is open, any reversecurrent will flow through the diode D10 so that the switch 554 does nothave to block more than the panel voltage of the photovoltaic panel 500.The solar cells between the bus bars 512 and 514 may be used as supplycells for powering a corresponding rapid shutdown control circuit duringsafety shutdown mode.

As previously noted, embodiments of the present invention are generallyapplicable to various solar cell and photovoltaic panel designs. Forexample, FIG. 13 shows a schematic diagram of a photovoltaic panel 500Ein accordance with an embodiment of the present invention. Thephotovoltaic panel 500E is a particular implementation of thephotovoltaic panel 500 of FIG. 5.

In the example of FIG. 13, the photovoltaic panel 500E comprises aplurality of series-connected solar cells 10. A solar cell 10 may be aconventional backside contact or front side contact solar cell. Each ofthe diodes D15-D18 serves as a protective bypass diode for acorresponding group of solar cells. In the example of FIG. 13, apositive node of the photovoltaic panel 500E is at the cathode of thediode D15 and a negative node of the photovoltaic panel 500E is at theanode of the diode D18. Similar to the photovoltaic panel 500B of FIG.9, in safety shutdown mode, the photovoltaic panel 500E shorts some ofthe solar cells 10 but leaves some of the solar cells 10 operational tocontinue providing power for a corresponding rapid shutdown controlcircuit.

In the example of FIG. 13, the switch 580 is normally open during normaloperation, thereby providing the panel voltage across the positive andnegative nodes. In response to detecting an initiation event, safetyshutdown mode is entered by closing the switch 580, thereby shorting outthe solar cells between the cathode of the diode D15 and the anode ofthe diode D17. During safety shutdown mode, the solar cells across thediode D18 are not shorted out and accordingly remains operational toprovide a supply voltage to a corresponding rapid shutdown controlcircuit.

FIG. 14 shows the photovoltaic panel 500A of FIG. 6 with a rapidshutdown control circuit for switching out solar cells in accordancewith an embodiment of the present invention. In the example of FIG. 14,a transistor Q1 (e.g., metal oxide semiconductor transistor (MOSFET)),serves as the switch device (see FIG. 8) for disconnecting thephotovoltaic panels 500A from the photovoltaic system in safety shutdownmode. In the example of FIG. 14, the rapid shutdown control circuit,which includes a controller 556 (e.g., a microcontroller) and a powersupply 557 (e.g., low dropout regulator (LDO)), is configured to controlthe conduction of the transistor Q1. In the example of FIG. 14, thetransistor Q1, the controller 556, and the power supply 557 share acommon reference. The power supply 557, which provides supply voltagefor powering the controller 556, receives an input supply voltage fromone or more solar cells of the photovoltaic panel 500A. The power supply557 may comprise a buck or boost converter depending on the input supplyvoltage received by the power supply 557 and the power requirements ofthe rapid shutdown control circuit. In the example of FIG. 14, the powersupply 557 receives an input supply voltage from the panel voltageacross the bus bars 510 and 514. The controller 556 turns on thetransistor Q1 during normal operation to provide the panel voltageacross the positive node 501 and the negative node 502. In response todetecting an initiation event, the controller 556 enters safety shutdownmode by turning off the transistor Q1 to disconnect the photovoltaicpanel 500A by removing ground reference from the negative node 502.

FIG. 15 shows a photovoltaic panel 500F in accordance with an embodimentof the present invention. The photovoltaic panel 500F is a particularimplementation of the photovoltaic panel 500 of FIG. 5. In the exampleof FIG. 15, similar to the photovoltaic panel 500B of FIG. 9, thephotovoltaic panel 500F provides supply cells between the bus bars 514and 515. Also, in the photovoltaic panel 500F, the wire 706 thatconnects the positive node 701 to a transistor Q2 is internal to thephotovoltaic panel 500F. The photovoltaic panel 500F is otherwise thesame as the photovoltaic panel 500B of FIG. 9.

In the example of FIG. 15, the transistor Q2 (e.g., MOSFET) serves asthe switch device for switching the solar cells in and out A rapidshutdown control circuit comprises a gate drive circuit 703 for drivingthe transistor Q2, a controller 705 (e.g., microcontroller), and a powersupply 704 for powering the gate drive circuit 703 and the controller705. In the example of FIG. 5, the power supply 704 receives an inputsupply voltage VIN from the supply cells between the bus bars 514 and515. The power supply 704 provides a supply voltage (e.g., 3.3V) to thecontroller 705 and another supply voltage (e.g., 12V) to the gate drivecircuit 703.

During normal operation, the controller 705 turns off the transistor Q2so that the panel voltage is provided across the positive node 701 andthe negative node 702. The panel current flows to a sense resistor R4.The controller 705 may monitor the voltage across the sense resistor R4to detect an initiation event, such as the panel current going down tozero. In response to detecting the initiation event, the controller 705enters safety shutdown mode by turning on the transistor Q2, therebyshorting out the solar cell from the bus bar 510 to the bus bar 514. Thecontroller 705 may detect a trigger release signal from the voltageacross the sense resistor R4. In response to detecting the triggerrelease signal, the controller 705 may turn off the transistor Q2 toresume normal operation.

In the example of FIG. 15, because the supply cells are in series withshorted solar cells, the transistor Q2 and the power supply 704 do notshare the same reference. Because of system voltage constraints, theinput supply voltage provided by the supply cells between the bus bars514 and 515 may be too low to generate adequate supply voltage for thegate drive circuit 503 and the controller 705. In that case, the powersupply 704 may have a boost topology.

FIG. 16 shows the photovoltaic panel 500B of FIG. 9 with a rapidshutdown control circuit in accordance with an embodiment of the presentinvention. In the example of FIG. 16, a transistor Q3 serves as theswitch device. In one embodiment, the transistor Q3 is a depletion-modeMOSFET, which is a normally-on switch device. Other normally-on switchdevices, such as a JFET, may also be used. To turn off the transistorQ3, the gate-to-source voltage VGS of the transistor Q3 is driven toapproximately −3V. In the example of FIG. 16, the transistor Q3 isdriven by a transistor Q4, which is a normally-off switch device, suchas an enhancement-mode MOSFET.

During normal operation, the transistor Q4 is on to provide agate-to-source voltage VGS of about −3V across the resistor R5 for thetransistor Q3, thereby turning off the transistor Q3 and providing thepanel voltage across a positive node 711 and a negative node 712. Inresponse to detecting an initiation event, safety shutdown mode isentered by turning off the transistor Q4 to turn on the transistor Q3,thereby shorting the solar cells between the bus bars 510 and 514. Thepower supply 714 (e.g., low dropout regulator) receives an input supplyvoltage from the solar cells between the bus bars 514 and 515 togenerate a supply voltage for the controller 713 (e.g.,microcontroller). The controller 713 controls the conduction of thetransistor Q4 to control the conduction of the transistor Q3.

In the example of FIG. 16, the controller 713 and the power supply 714share a common reference with the transistor Q4 to simplify the powersupply design. Depending on the input supply voltage provided by thesupply cells, the power supply 714 may employ boost topology to generateadequate supply voltage for the controller 713. In the example of FIG.16, using a normally-on switch device to short the solar cells in safetyshutdown mode has the advantage of defaulting to a safe state when powerto the rapid shutdown control circuit is lost.

Generally speaking, a diode in the above-described photovoltaic panelsmay be replaced by a switch device, such as a MOSFET. A switch devicefor switching solar cells in and out of the photovoltaic system may beimplemented using a MOSFET, junction gate field effect transistor(JFET), insulated gate bipolar transistor (IGBT), or other types oftransistors. Normally-on type transistors, such as depletion-modeMOSFETs or JFETs, are especially beneficial in shorting implementationsas these transistors will revert to shorting the solar cells (andlowering the panel voltage) in the event of rapid shutdown controlcircuit malfunction or power failure. In disconnect implementations, thetransistor and diode (or another transistor) may be used to form a buckconverter and track the photovoltaic panel's maximum power point.

Solar cells may be switched out from the photovoltaic system by shortingthe solar cells or disconnecting the solar cells from the photovoltaicsystem. In embodiments where shorting is employed, the rapid shutdowncontrol circuit and the switch device may be integrated into thephotovoltaic panel to prevent the rapid shutdown control circuit andswitch device from being accidentally removed, thereby posing a safetyissue where the solar cells cannot be shorted out for rapid shutdown.With disconnect embodiments, the solar cells will be disconnected whenthe rapid shutdown control circuit and switch device are accidentallyremoved, which simply lowers the system voltage (i.e., output voltage ofthe photovoltaic system). For redundancy in shorting embodiments, twonormally-on switch devices may be put in parallel. For redundancy indisconnect embodiments, two normally-off switch devices can be put inseries. A rapid shutdown control circuit may receive an enable signalfrom the inverter or a main controller of the photovoltaic system duringnormal operation, and initiate rapid shutdown when the enable signal islost. The enable signal may be received over a wired connection,wireless connection, or some other communication channel.

FIG. 17 shows a flow diagram that schematically illustrates operation ofa photovoltaic system in accordance with an embodiment of the presentinvention. In the example of FIG. 17, the photovoltaic system includesan inverter 610 and a plurality of photovoltaic panels 500 (e.g., 500,500A, 500B, etc.). In the example of FIG. -17, two strings ofseries-connected photovoltaic panels 500 are connected in parallel. Thenumber of photovoltaic panels 500 and their arrangement in thephotovoltaic system may vary depending on the application.

In the example of FIG. 17, the photovoltaic system may start in a normaloperating mode 621, wherein the inverter 610 draws forward current 631from the photovoltaic panels 500. A switch 611 is closed when thephotovoltaic system is in the normal operating mode 621. Correspondingrapid shutdown control circuits may detect the forward current 631 toenter normal operating mode 621.

The photovoltaic system may transition 601 to enter safety shutdown mode622 in response to detecting an initiation event, which indicatesinitiation of rapid shutdown of the photovoltaic system. The initiationevent may be the system current (which is detected by a rapid shutdowncontrol circuit as the panel current) dropping down to zero. The systemcurrent may drop down to zero when the inverter 610 is shutdown, whichis represented by opening the switch 611. Rapid shutdown controlcircuits may detect the initiation event to enter safety shutdown mode622.

In the example of FIG. 17, the photovoltaic system may transition 602 toreverting mode 623 to return to normal operation. The switch 611 isclosed in the reverting mode 623 to indicate that the inverter 610 isready to revert back to the normal operating mode 621 to resume normaloperation. In the example of FIG. 17, to revert back to the normaloperating mode 621, the inverter 610 feeds a reverse current 632 to thephotovoltaic panels 500. The rapid shutdown circuits detect the reversecurrent 632 and, in response, transitions 603 to the normal operatingmode 621. The inverter 610 may also draw forward current 631 from thephotovoltaic panels 500 to indicate resumption to the normal operatingmode 621. That is, drawing the forward current 631 or feeding thereverse current 632 may serve as a trigger release signal for resumingnormal operation.

In the example of FIG. 17, two strings of series-connected photovoltaicpanels 500 are connected in parallel. In some installations, current maycirculate between strings of photovoltaic panels when the inverter isshutdown. The circulating current may be due to panel mismatch, shading,degradation, or other reasons. In that case, it may be difficult todetect the mode of the photovoltaic system from the string current. Inthose cases, a different release signal may be employed or thephotovoltaic system may be limited to a single string ofseries-connected photovoltaic panels.

FIG. 18 shows a schematic diagram of a photovoltaic system in accordancewith an embodiment of the present invention. In the example of FIG. 18,the photovoltaic system includes strings 651 and 652 of series-connectedphotovoltaic panels and the photovoltaic inverter 610. In the example ofFIG. 18, the photovoltaic panels 500 (and thus the solar cells therein)are switched in and out of the photovoltaic system by disconnection aspreviously described. In the example of FIG. 18, a diode 653 is acrosseach photovoltaic panel 500 to prevent circulating current when thephotovoltaic system is in safety shutdown mode due to rapid shutdown.That is, the diodes 653 block reverse currents, thereby preventingcirculating current from flowing from one string to another. As aparticular example, the diodes 653 of the string 652 block circulatingcurrent 654 from the string 651.

FIG. 19 shows a schematic diagram of a photovoltaic system in accordancewith an embodiment of the present invention. In the example of FIG. 19,the photovoltaic system includes strings 661 and 662 of series-connectedphotovoltaic panels and the photovoltaic inverter 610. In the example ofFIG. 19, the photovoltaic panels 500 (and thus the solar cells therein)are switched in and out of the photovoltaic system by shorting aspreviously described. In the example of FIG. 19, a diode 663 is inseries with each string to prevent circulating current when thephotovoltaic system is in safety shutdown mode due to rapid shutdown.That is, the diodes 663 block reverse currents, thereby preventingcirculating current from flowing from one string to another. As aparticular example, the diode 663 of the string 662 block circulatingcurrent 664 from the string 661. The circulating current 664 may be fromsupply cells or other solar cells that are not switched out in safetyshutdown mode.

FIG. 20 shows a schematic diagram of a photovoltaic system in accordancewith an embodiment of the present invention. In the example of FIG. 20,the photovoltaic system includes strings 671 and 672 of series-connectedsolar cells 678 and the photovoltaic inverter 610. The solar cells 678of the string 671 are switched in and out of the photovoltaic system byshorting, and the solar cells 678 of the string 672 are switched in andout of the photovoltaic system by disconnection. The solar cells 678form a first group of solar cells, and the solar cells 677 form a secondgroup of solar cells. The solar cells 677 serve as supply cells forproviding power to rapid shutdown control circuits. In one embodiment,the solar cells 677 also provide auxiliary power (e.g., approx. 30V) tothe photovoltaic panel 610 to power inverter control circuitry. This isadvantageous because the AC grid may not be able to provide power to thephotovoltaic inverter 610 during rapid shutdown events.

FIG. 21 shows a schematic diagram of a photovoltaic system in accordancewith an embodiment of the present invention. In the example of FIG. 21,the photovoltaic system includes strings 681 and 682 of series-connectedphotovoltaic panels and the photovoltaic inverter 610. In the example ofFIG. 21, the photovoltaic panels 500 (and thus the solar cells therein)are switched in and out of the photovoltaic system by shorting aspreviously described. In the example of FIG. 21, a diode 683 is inseries with each string to facilitate detection of the release triggerthat indicates resumption of normal operation. More particularly, toresume normal operation, the photovoltaic inverter 610 may generatereverse current to charge the capacitance of the array of photovoltaicpanels 500. The resulting array voltage is distributed to all of thephotovoltaic panels 500, and each photovoltaic panel 500 would detect anincrease in the panel voltage. Rapid shutdown control circuits maydetect this voltage increase as a release trigger to resume normaloperation by removing the shorts across the photovoltaic panels 500.

Photovoltaic systems with rapid shutdown have been disclosed. Whilespecific embodiments of the present invention have been provided, it isto be understood that these embodiments are for illustration purposesand not limiting. Many additional embodiments will be apparent topersons of ordinary skill in the art reading this disclosure.

What is claimed is:
 1. A photovoltaic system comprising: a photovoltaicpanel comprising a first group of solar cells, the photovoltaic panelbeing configured to generate a panel voltage and a panel current; and acontrol circuit configured to detect initiation of a rapid shutdown ofthe photovoltaic system by detecting a shutdown of a photovoltaicinverter, to lower the panel voltage below a safety level in response todetecting initiation of the rapid shutdown by switching out the firstgroup solar cells, to monitor a line that connects the panel voltage tothe photovoltaic inverter for a release trigger signal indicatingresumption of normal operation, and to switch back the first group ofsolar cells to restore the panel voltage back to a normal operatinglevel in response to detecting the release trigger signal on the line.2. The photovoltaic system of claim 1, wherein the photovoltaic panelfurther comprises a second group of solar cells that are not switchedout in response to detecting the initiation of the rapid shutdown, andthe control circuit is powered by the second group of solar cells, andwherein the release trigger signal is a reverse current generated by thephotovoltaic inverter, and wherein the reverse current flows in adirection opposite to that of the panel current.
 3. The photovoltaicsystem of claim 1, wherein the control circuit comprises a switch deviceand the control circuit controls the switch device to short out thefirst group of solar cells in response to detecting the initiation ofthe rapid shutdown.
 4. The photovoltaic system of claim 3, wherein theswitch device is a normally-on transistor.
 5. The photovoltaic system ofclaim 1, wherein the control circuit comprises a switch device and thecontrol circuit switches out the first group of solar cells bycontrolling the switch device to disconnect the first group of solarcells from the photovoltaic system.
 6. The photovoltaic system of claim5, wherein the switch device is a transistor, and the control circuitturns off the transistor to disconnect the first group of solar cellsfrom the photovoltaic system.
 7. The photovoltaic system of claim 1,wherein the photovoltaic inverter is configured to send the releasetrigger signal to the control circuit.
 8. The photovoltaic system ofclaim 7, wherein the release trigger comprises a ripple voltageinitiated by the photovoltaic inverter.
 9. The photovoltaic system ofclaim 1, wherein the control circuit detects the initiation of the rapidshutdown by detecting the panel current dropping to zero and remainingat zero for at least a predetermined time.
 10. A method of operating aphotovoltaic system, the method comprising: monitoring a panel voltageand a panel current of a photovoltaic panel; detecting initiation of arapid shutdown of the photovoltaic system; in response to detectinginitiation of the rapid shutdown, entering safety shutdown mode byswitching out a group of solar cells of the photovoltaic panel to lowerthe panel voltage below a safety level; monitoring a line that connectsthe panel voltage to a photovoltaic inverter for a release triggersignal that indicates-resumption of a normal operation; and in responseto detecting the release trigger signal, resuming the normal operationby switching back the group of solar cells to restore the panel voltageto a normal operating voltage level above the safety level.
 11. Themethod of claim 10, wherein switching out the group of solar cellscomprises shorting out the group solar cells.
 12. The method of claim10, wherein switching out the group of solar cells comprisesdisconnecting the group of solar cells.
 13. The method of claim 10,further comprising: during the safety shutdown mode, powering a controlcircuit that monitors the line for the release trigger signal usinganother group of solar cells, wherein the release trigger signal is areverse current generated by the photovoltaic inverter and wherein thereverse current flows in a direction opposite to that of the panelcurrent.
 14. The method of claim 10, wherein detecting the initiation ofthe rapid shutdown comprises: detecting that the panel current drops tozero.
 15. The method of claim 10, wherein monitoring the line comprises:monitoring the line for presence of a ripple voltage.
 16. The method ofclaim 10, wherein monitoring the line comprises: monitoring the line forcurrent initiated by the photovoltaic panel.
 17. A control circuit for aphotovoltaic panel of a photovoltaic system, the control circuitcomprising: a switch device that is coupled to switch a first group ofsolar cells in and out of the photovoltaic panel; and a controller thatis configured to detect initiation of a rapid shutdown of thephotovoltaic system, to control the switch device to lower the panelvoltage below a safety level in response to detecting initiation of therapid shutdown by switching out the first group of solar cells, tomonitor a line that connects the panel voltage to a photovoltaicinverter for a release trigger signal indicating resumption of normaloperation, and to control the switch device to restore the panel voltageback to a normal operating level in response to detecting the releasetrigger signal by switching in the first group of solar cells.
 18. Thecontrol circuit of claim 17, wherein the switch device comprises anormally-on transistor.
 19. The control circuit of claim 17, wherein thecontrol circuit is configured to detect the initiation of the rapidshutdown by monitoring a panel current of the photovoltaic panel anddetecting the initiation of the rapid shutdown when the panel currentdrops to zero.
 20. The control circuit of claim 17, wherein the releasetrigger signal comprises a ripple voltage initiated by a photovoltaicinverter.